1. Field of the Invention
The present invention relates to an image capturing apparatus having an auto-focusing function.
2. Description of the Related Art
There is conventionally known a technique of performing focus detection at high speed and accuracy by using a focus detection method adopting a phase difference detection method in an image capturing apparatus including an imaging optical system and an image pickup device. In a focus detection technique using the phase difference detection method, a pupil division unit divides a light beam radiated from the imaging optical system into at least two areas, and the light beam of each area is photoelectrically converted to obtain a pair of focus detection signal strings of two images. A focus shift amount in a predetermined focal plane, that is, a defocus amount is detected from the relative image shift amount between the two signal strings. In focus detection using the phase difference detection method, in an in-focus state, the strengths of the signal strings of the two images coincide with each other, and the relative image shift amount should also be zero. However, due to vignetting of the focus detection light beam caused by the imaging optical system and various aberrations of the imaging optical system, the coincidence of the two images deteriorates, resulting in a focus detection error. Due to vignetting and various aberrations, the proportional relationship between the defocus amount and the relative image shift amount between the two images deteriorates. To perform focus detection at high accuracy, therefore, it is necessary to eliminate the errors. A technique for this purpose has also been proposed.
On the other hand, there has been proposed a technique in which a two-dimensional CMOS sensor or the like is used as an image pickup device to arrange focus detection pixels for phase difference detection on the sensor. An image capturing pixel and a focus detection pixel are arranged on the same plane. Therefore, when the image capturing pixel is in the in-focus state, the focus detection pixel is also in the in-focus state. For this reason, in principle, no relative image shift occurs between two images for phase difference detection in the in-focus state, and thus an error hardly occurs. Since, however, the focus detection pixel includes two photoelectrical conversion portions, a circuit for reading out accumulated pixel signals is complicated. A method of suppressing complication of a circuit for reading out pixel signals is described in Japanese Patent Laid-Open No. 2008-103885.
In the technique described in Japanese Patent Laid-Open No. 2008-103885, each of a plurality of focus detection pixels each including two photoelectric conversion portions outputs a signal obtained by adding output signals from the two photoelectric conversion portions.
According to Japanese Patent Laid-Open No. 2008-103885, however, when a value outputtable from the focus detection pixel is exceeded (saturated), crosstalk occurs due to leakage of charges between the two photoelectric conversion portions of the focus detection pixel. Crosstalk causes an output signal to include a signal in addition to a signal obtained by photoelectrically converting the light beam from the imaging optical system, thereby disabling correct focus detection.